The stability of the surface oxide film present on aluminum is a definite impediment to fabricating aluminum parts, such as by brazing. In brazing the film acts as a barrier to wetting and flow of the filler metal required for joint formation. Oxide removal and prevention of reoxidation are the principal requirements for a successful aluminum joining method.
Fluxless brazing has assumed a position of commercial importance because it does not require the removal of a flux residue and reduces the susceptibility to aqueous corrosion of parts having this flux residue. One of the first innovations to promote better fluxless brazing, was the discovery that magnesium, when incorporated as part of the filler metal, promotes the wetting of the oxide by the filler metal. However, the presence of gaseous species of water and oxygen within the brazing chamber react with the promoter element (such as magnesium) to maintain the oxide barrier by inhibiting oxide/filler metal wetting and/or building the oxide thickness to the extent that the barrier envelopes the liquid filler metal and prevents wetting and flow. This is particularly troublesome with inert atmospheres but it also occurs to a lesser degree in vacuum brazing. It has not been fully recognized by the prior art of the disturbance to brazing that results from the presence of even small amounts of gaseous species of O.sub.2 and H.sub.2 O. For example, when a furnace is evacuated to a vacuum level of about 10.sup.-4 -10.sup.-5 Torr and simultaneously heated to a temperature in excess of the melting point of the filler metal containing magnesium, it has been found that trace gaseous species of oxygen and water remain in existence within the vacuum environment. When porosity develops in the oxide film slightly prior to the evolution of magnesium from the solid filler metal, the gaseous species of O.sub.2 and H.sub.2 O react with the oxide film and form more oxide or a duplex oxide. This subsequently prevents the promoter or magnesium from reacting properly with the original oxide film when porosity develops to a greater extent. As a consequence, the oxide film floats on top of the fluidized filler metal and little or no wetting of the base metal occurs.
Technology to date has not been able to provide a method which would meet these three criteria simultaneously and thereby promote a high quality braze when fabricating the aluminum parts within an inert atmosphere which may contain considerably more gaseous species of oxygen and water than that which would be contained in a vacuum chamber. The requirement of an inert atmosphere is placed upon this invention so that the economy of carrying out such brazing process is increased and thereby the versatility and application of the brazing method can be extended to high volume parts requiring minimum costs.
There are three physical characteristics that must occur simultaneously for optimum results to occur in wetting and formation of a filler at the braze joint free of oxides. These three physical characteristics consist of (a) the transport of magnesium from the filler metal or other source to the oxide film interface in the correct amount and at the precise moment for that which is needed for wetting purposes and not later nor earlier and no more nor less than so needed, (b) the formation of porosity within the oxide film at the right time in the brazing sequence, roughly desirable only after the filler metal melts and the magnesium promoter is available at the oxide interface, and (c) the melting of the filler metal at a sufficiently low enough temperature to correspond with the earliest formation of porosity in the oxide film, and (d) wetting of the oxide by the filler metal.